Transparent conducting oxide films (TCOs) are used extensively for a variety of applications including but not limited to architectural windows, flat-panel displays, thin-film photovoltaic devices, electrochromic windows, and polymer-based electronics. The electrical and optical properties of most TCOs fabricated by conventional techniques are adequate for many applications, but further improvement in both conductivity and transparency are desirable for TCOs used in semiconductor device fabrication.
Conductive SnO2, typically doped with fluorine, is a popular TCO commonly used on low-emissivity windows and in solar cells as a front contact. SnO2 has a relatively high optical bandgap in the range of 3.62 eV to 4.0 eV and moderate electron mobilities of about 13 cm2V−1 s−1 to 35 cm2V−1 s−1. CdO is another known TCO. Unlike SnO2, CdO is not widely used due to the narrow optical band gap of CdO and the toxicity of cadmium. However, CdO has demonstrated electron mobilities that are five to ten times higher than other commercially available TCO materials. Electron mobility is a critical parameter for TCO materials used in semiconductor devices. High electron mobility will improve both the electronic and optical properties of a TCO material for many uses.
Cadmium stannate, Cd2SnO4, is comprised of a 2:1 atomic ratio mixture of CdO and SnO2 that is known to have unusually high electron mobility as well. Cd2SnO4 films also have low optical absorption in relevant wavelengths and high electrical conductivity. For example, a Hall mobility of over 60 cm2V−1 s−1 has been reported in thin-film Cd2SnO4 produced by radio frequency sputtering. In this previous report, charge carrier concentrations of 5×1018 cm−3 have been achieved in single-phase spinel-type Cd2SnO4 films which correspond to the observed high Hall mobility measurements. Several models have been proposed to explain the high electron mobility of spinel Cd2SnO4 including models indicating that the high electron mobility of Cd2SnO4 might be due to the crystal structure of this material. These favorable properties have made crystalline Cd2SnO4 potentially suitable for a wide range of semiconductor device fabrication applications and other applications.
Unfortunately, several challenges are presented by the large-scale manufacture of spinel Cd2SnO4 films. In particular, the processes that may be used to produce high-quality Cd2SnO4, for example radio frequency or magnetron sputtering plus high-temperature proximity heat treatment are not suitable for large-volume manufacture. In addition, the temperatures required for selected manufacturing steps prohibits the deposition of spinel or other crystalline Cd2SnO4 films on inexpensive substrates such as soda-lime glass that are desirable for high-volume production.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.